1. Field of the Invention
The present invention concerns the dimensional measurement of workpieces that undergo variations in dimension with changes in temperature. The workpieces are measured by gauges that themselves undergo variations in dimension with changes in temperature. The present invention more particularly concerns measurement systems that compensate for the temperature-induced dimensional variation of workpieces, and/or the temperature-induced dimensional variation of gauges, in measuring the dimensions of workpieces with gauges.
2. Background of the Invention
2.1 The Effect of Temperature on Dimensional Measurement
It is known that certain materials, particularly metals, expand and contract with temperature. This expansion or contraction is normally expressed as a coefficient of expansion. Such a coefficient of expansion is expressed in terms of dimension per unit dimension per degree temperature. For example, a coefficient of expansion on the order of 0.000007 inches per inch per degree Fahrenheit is typical for certain ferrous metals.
Dimensional changes with temperature obviously mean that a metal workpiece does not measure the same dimensions at different temperatures. This makes it imprecise to determine the true, temperature-normalized, dimensions of the workpiece unless the workpiece is brought to a standard, reference, temperature. This standard, reference, temperature at which workpieces are measured is 59.degree. F. by convention.
It is not always convenient to measure the dimensions of a workpiece while it is at the predetermined, reference, 59.degree. F. temperature. The workpiece may be either hotter or colder than the reference temperature and may correspondingly exhibit expanded or contracted (or vice versa) dimensional measurements. If the accuracy of quantitative dimensional measurement, or the accuracy of qualifying a workpiece to some dimensional standard, is critical, then it is either necessary (i) to bring the workpiece to the predetermined, reference temperature at which the measurements may be performed, (ii) to compensate for the effect of the difference between the actual workpiece temperature and the reference temperature on the dimension(s) of the workpiece or (iii) to have a pre-determined dimensional reference "standard" that is both at the temperature of the part to be measured and that is also of the same material, and to measure the difference between the "standard" and the part.
If the workpiece has considerable thermal mass, and/or it is not readily subject to be adjusted in temperature, and/or a standard of identical material is not available, then it may be more efficient, or even mandated, that the workpiece should be measured at its existing temperature. That dimension which the workpiece would exhibit should it be brought to a predetermined, reference, temperature may then be calculated. This calculation requires knowledge of (i) the measured dimension, (ii) the workpiece actual temperature, and (iii) the thermal coefficient of expansion for the material of the workpiece.
2.2 Previous Temperature-Compensated Dimensional Measurement Systems
One previous example of a temperature-compensated dimensional measuring system is shown in U.S. Pat. No. 3,594,909 for an APPARATUS FOR MEASURING A DIMENSION OF A MEMBER to Schultz. The workpiece being measured is, for example, a wide-flange beam being formed in a rolling mill. The flange is measured while it is still near its rolling temperature of approximately six hundred degrees Fahrenheit (600.degree. F.). The temperature-compensated dimensional measuring system taught within the Schultz patent obviates the need for permitting a sample piece of the beam to cool to room ambient temperature before its dimensions and/or symmetry may be checked for unacceptable deviations from normal. In the apparatus of Schultz, the temperature measurement means is a thermocouple probe. The probe contacts a workpiece that is elevated in temperature. A computer receives the signals from this probe and calculates the temperature-adjusted workpiece measurements. These measurements are, however, displayed only somewhat inexactly as a trace registered upon a strip recorder. Manual monitoring of the traces being recorded upon the strip recorder allows recognition of the conformity or nonconformity of the workpiece (typically a hot rolled I-beam) to symmetry and dimensional standards.
The previous temperature compensation of Schultz is primarily directed to producing and displaying an output signal representative of dimension upon a strip recorder when this output signal will be within a predefined range, and when the resulting trace will be observable for acceptable or unacceptable deviations within this range, regardless of variations in temperature of the workpiece for which dimension is being sensed. Schultz is concerned with checking I-beams for conformance with design standards; i.e., so that each I-beam will carry its design load and will mate with other I-beams of the same specification. Schultz is not concerned with dimensional measurement supporting the precision fitting of parts, as is the present invention.
Schultz is not concerned with precise quantitative dimensional measurement of a workpiece while the workpiece is at a displacement temperature from a predetermined, reference, temperature. Schultz is not concerned that a measured dimension may be converted to that temperature-normalized dimension which a workpiece would exhibit should it be brought to the reference temperature. The "compensation" of Schultz is basically an "accounting" for rather large dimensional deviations of a steel beam workpiece when it is still near its rolling temperature of about 1600.degree. F.
The present invention is concerned with precise quantitative compensation of accurate (typically .+-.0.001 inches) dimensional measurements of a workpiece when the temperature of the workpiece over a wide range (typically 70.degree. F..+-.50.degree. F.) is accurately known (typically within .+-.1.degree. F.). The reason that the present invention is so concerned is not simply to garner measurement numbers. The present invention is directed to better enable a part "A" to mesh or fit into part "B". This simple concept is important. If at 59.degree. F. a hole "A" is 2.0000.+-.0.0001 inches in diameter, and if at the same 59.degree. F. a shaft "B" of the same material is 1.9990.+-.0.0001 inches in diameter, then the shaft will (and not just "should") fit within, and always fit within, the hole. It is of great benefit to know that things will mesh or fit together, and to know how well things will mesh or fit together, as hereinafter explained.
2.3 The Requirement for Temperature-Compensated Dimensional Measurement
The need for temperature-compensated quantitative dimensional measurement is greater than is commonly recognized. Commonly available measurement reference standards, typically ground steel, are accurate to .+-.0.00001 inch. The dimensions of some workpieces, such as the journals of railroad axles hereinafter discussed, are augmented by process of plating to accuracies of .+-.0.00005 inches. These same journals are diminished by process of machining to accuracies of .+-.0.0001 inch. (The journals may of course be machined undersize and plated back to a desired dimension.)
This degree of dimensional accuracy is common. Yet these accuracies are completely overwhelmed by any difference in workpiece temperature, and/or the temperature of the frame of the measuring tool that measures the workpiece, from a reference temperature. As mentioned, steel has a coefficient of expansion on the order of 0.0000068 inches per inch per degree Fahrenheit. Consequently, for a workpiece larger than 1 inch an uncompensated temperature variation of 10.degree. F. or more can be the single greatest source of measurement inaccuracy.
Uncompensated dimensional variation due to temperature variation has been, in the opinion of the inventor, a prime driver in the setting of dimensional standards to which larger mechanical parts such as journals and bearings are commonly constructed. The nominal .+-.0.001 inch tolerance to which these parts (such as railroad axles, discussed hereinafter) are sized is not the tolerance at which reasonably optimized, let alone best, mechanical wear performance is obtained. Mechanical wear of parts at tolerances of .+-.0.0001 inch is markedly better than at lessor tolerances, especially with modern lubricants. For example, automobile manufacturer Volvo of Sweden has gained a reputation for durability of automotive mechanical parts which may be due, in part, to the little-recognized fact that the parts of this manufacturer are reportedly machined to better dimensional accuracy than the Society of Automotive Engineers (SAE) standards followed by the domestic U.S. automotive industry.
If higher dimensional accuracies are beneficial, and realizable, then why are these higher accuracies not common? Why are the dimensional standards of industry so liberal? A primary cause as to why higher dimensional accuracies than current standards are not cost effectively realizable is the difficulty in cost effective accurate control of temperature. Accurate, .+-.1.degree. F., control of ambient temperature is typically difficult and expensive. Worse, a tight control of ambient temperature does not invariably guarantee tight control of a workpiece temperature because the workpiece temperature may be affected by heats of machining and other processing. Finally, temperature stabilization of workpiece thermal masses takes time, and time is equivalent to cost in a production environment.
As well as the sensitivity of dimensional measurements of workpieces to temperature changes, it should be well understood that the gauges and tools of industry are themselves generally suffering much greater dimensional variation due to temperature change during use than any other single factor to which the gauges or tools are commonly subject. That the operative heads of drilling and milling machines change size with variation in their temperature is obvious. But even such tool faces as the cutter knife of machine that cuts veneer from a log are unexpectedly subject to undesirable change with temperature. A heat-expanded veneer cutter knife must cut a thicker wood veneer (else the cold knife would cut the veneer too thin), reducing the amount of veneer that can be produced from a log. Undesired dimensional variation with temperature change thus directly translates into reduced production of veneer. The veneer produced also exhibits an undesirable increased variability of thickness. This is but one of the more obscure of many examples that generally show that dimensional variation due to temperature change is an underecognized cost driver both during production and during life cycle use of the products produced.
2.4 An Example of a Dimensional Measurement Problem Strongly Affected by Temperature Variation
One particular example of a workpiece upon which it is desired to obtain precise dimensional measurements while it is at a temperature differential from a predetermined, reference, temperature is the axle of a railroad car. The axle of a railroad car, typically made of steel, is both large and heavy. It exhibits a large thermal mass. The axles are normally received into an indoors test environment from the out-of-doors at temperatures which, in most regions of the country during most portions of the year, are distinctly neither at room temperature (typically 73.degree. F.) nor the reference temperature (nominally 59.degree. F.).
The dimensions of the axle must be determined at a reference temperature, nominally fifty-nine degrees Fahrenheit (59.degree. F.). If the axles have become warmer or colder than this reference temperature by exposure to the environment, then a delay of many hours, or days, would be encountered if the axles were to be permitted to thermally stabilize at a 59.degree. F. ambient temperature.
Because of these problems with temperature stabilization of railroad car axles, the axles are typically not quantitatively measured, but are rather only compared to a reference gauge, or "Jo", block. The Jo block is of the identical material to the axle and is of a known dimension. This dimension is the nominal standard for the journal of a railroad car axle. A number of axles subject to comparison are equalized at the same temperature as the "Jo" block, typically at the room temperature of the test environment. The temperature equalization is normally aided by fans that blow air over both the "Jo" block and the axles for an extended period, typically overnight.
In order to compare the axles a snap gauge measurement tool is first zeroed to the "Jo" block. The tool is then used to measure a plus (+) or minus (-) size differential of an axle. The process continues with repetitive rezeroings and measurements. Ultimately the tolerances of the axles relative to the "Jo" block are known even if the precise quantitative dimensions of the axles are only but imperfectly known.
The utility of comparing an object to be undersize or oversize relative to a reference is not as useful as knowing the exact, temperature-normalized, dimensions of such object. For example, the diameter of an axle of a railroad car is typically desired to be quantitatively measured to within .+-.0.001 inch. This accuracy in measurement is necessary to determine whether axles are (i) repairable to be within required normal dimensional range, (ii) issuable for use by being qualified to be within normal dimensional range, or (iii) subject to scrappage for being of unrepairable dimensions. Typically, a railroad axle larger than 6.1915 inches in diameter is too large, but repairable. An axle diameter between 6.1915 and 6.1905 inches is within the acceptable tolerance range. An axle diameter between 6.1885 and 6.1905 inches is slightly too small, but normally repairable. An axle diameter below 6.1885 inches is unsuitable for repair or subsequent use.
The stringent requirement that the axle of a railroad car should exhibit a diameter of 6.190 plus or minus 0.0005 inches is due to the fact that dimensional mismatch between the axle and its bearing can result, at the high loads to which railroad cars are subject, excessive rolling friction. Such rolling friction results in thermal build-up and possible catastrophic failure of the axle and/or axle bearings. In the extreme case this can result in derailments. It is believed that as many as 60% of the catastrophic failures of railroad car bearing axle assemblies may be traceable to out of tolerance bearing race or axle journal conditions.
Meanwhile that the relatively large railroad car axle must be measured very precisely, each change in temperature of this axle of twelve degrees Fahrenheit (12.degree. F.) causes a variation in the shaft diameter of approximately 0.0005 inches, or fully one-half of the total 0.001 inch tolerance range within which the axle must be dimensionally qualified! (Explicitly, 6.190 inches.times.0.0000068 inches per inch per .degree.F..times.12.degree. F.=0.0005051.)
Without temperature-compensated measurement, it is obviously necessary not only that an axle should be brought approximately to the predetermined, reference measurement temperature, but that, indeed, the axle should be brought very precisely to this temperature. The aforementioned dimensions are those that the standard railroad axle must exhibit at precisely fifty nine degrees Fahrenheit (59.degree. F.). It is hard to make a large thermal mass railroad car axle assume, and hold, this precise temperature. Accordingly, some axles are sent for rework, and some are even rejected, incorrectly. Conversely, and more detrimentally, certain axles for which the dimensions are improper may otherwise be certified for use.
Accordingly, the precise measurement of large, dimensionally thermally sensitive objects such as railroad car axles with high quantitative accuracy typically requires performing and reperforming the measurement process. Typically, a railroad car axle is measured several times before a confidence level can be developed that the correct measurements have actually been registered. This is obviously inefficient. Additionally, it is unsound safety practice that the measurement process should be so extremely dependent upon temperature variation that a difference in the temperature of the workpiece of a mere 48 degrees Fahrenheit (48.degree. F.) (corresponding to the difference between 6.1905 inches and 6.1885 inches) might cause an axle that is correctly subject to permanent scrappage to instead be issued directly for use without rework! An improved device for the quantitative dimensional measurements of workpieces that are dimensionally sensitive to temperature change is required.